The discovery of high-temperature ceramic superconductors in 1986 ignited an ongoing interest in these materials and their unique properties relating to electric-current conduction and magnetism. When a high-temperature superconductor (HTS) is cooled below its transition temperature, the HTS can transmit a limited amount of electric current with little or no resistance. While in the superconducting state, the superconductor also exhibits promising magnetic mirroring properties which, for example, enable magnetic materials to be levitated above it.
Those superconductors with the highest known heat tolerances must nevertheless be cooled to about 125 K or less to operate satisfactorily as a superconductor. Such refrigeration is often achieved with liquid nitrogen for some materials and with liquid helium where lower temperatures are desired.
Superconducting magnets operated in liquid helium are useful in a variety of applications, such as magnetic resonance imaging, energy storage rings, particle accelerators and maglev trains. The superconducting magnets are energized by current leads with temperatures spanning from 300 K at a warm end to 4.2 K at a cold end where the current lead is coupled with the superconducting magnet.
One type of current lead generally used to energize a superconductingg magnet operated in a bath of liquid helium boiling at 4.2 K is made of copper and has a cross sectional area xe2x80x9coptimizedxe2x80x9d for a given lead length and operating current to minimize the heat flowing into the liquid helium. Each of these xe2x80x9cconventionalxe2x80x9d leads injects heat into liquid helium at a rate of approximately 1 mW/A. Thus, a conventional 5-kA lead with a heat input of 5 W boils off liquid helium at a rate of 7.2 liter/hr. ({dot over (m)}he=0.25 g/s), or a boil-off rate of about 15 liter/hr. for a pair of leads. In terms of the compressor power required to maintain the liquid helium level in the magnet system, a 5-W plower input at 4.2 K can require a minimum of about 2,500 W for a large cryogenic system to as much as 10,000 W for most smaller systems.
Others have used current leads comprising HTS tapes, each tape typically consisting of an HTS layer on a metal substrate. While each of the tapes, alone, has a limited or xe2x80x9ccriticalxe2x80x9d current level that it can carry through the HTS in a superconducting mode, the critical current capacity of the lead is additively increased by bonding a sufficient number of HTS tapes (often several hundred) together in a single lead. The number of tapes and other parameters of the lead are selected such that the entirety of the HTS section of the lead can carry the all of its rated transport current in a superconducting mode.
A method of this invention uses a current lead comprising a high-temperature superconductor (HTS) component and a normal conductive component operating in a current-sharing mode under normal operating conditions to energize a superconducting device. In accordance with this method, the current lead is coupled to the superconducting device, and electric current is delivered through the current lead at an amperage above the critical-current-carrying capacity over a region extending from the warm end of the HTS component, meaning that the current is carried not only by the HTS component but also by the normal conductive component. Because of this current-sharing by the normal conductive component, joule heating is dissipated in the region.
xe2x80x9cNormalxe2x80x9d conduction is defined as conduction in a material, such as a room-temperature conductive metal, with more than trace levels of resistance. The xe2x80x9cnormal operating conditionsxe2x80x9d under which the current lead is operated are defined as the typical, intended and roughly-equilibrated cooling and conductance conditions under which the lead operates to supply the necessary current to the superconducting device for a sustainable period of time. These conditions can be contrasted with aberrant or fault conditions which may be predicated, for example, by an unexpected power surge or an emergency loss of coolant.
When all or a part of the HTS section of a current lead operates in the current-sharing mode, the current lead operates with significantly enhanced efficiency over a conventional copper lead. At least two benefits are also provided over the use of an HTS lead operating entirely as a near-zero-resistance superconductor. First, the amount of HTS that is needed is reduced, thereby reducing the current lead""s capital cost of production without sacrifice to reduction in the cold end heat load in comparison with that achievable by a xe2x80x9cconventionalxe2x80x9d HTS current lead of the same current rating. Second, the operation of the lead in the current-sharing mode nevertheless ensures stable operation, making the lead well-protected when operated in accordance with this invention.